The present invention relates to an igniter for controlling a current supplied to a primary winding of an ignition coil for an internal combustion engine.
An example of a known igniter is illustrated in FIGS. 6 and 7 which are a circuit diagram and a partially cut-away plan view thereof, respectively. In FIG. 6, the known igniter, generally designated by reference character I, includes a power transistor circuit 1 for turning on and off the current supply to a primary winding 2a of an ignition coil 2 which has a secondary winding 2b connected through an unillustrated distributor to a plurality of spark plugs (not shown), a current detector 3 in the form of a resistor connected between the power transistor circuit 1 and ground for detecting a voltage thereacross developed by a current flowing from a power supply 4 to ground through the primary winding 2a and the power transistor circuit 1 when it is turned on, and a current limiter 5 connected through a resistor 9a to a junction between the power transistor circuit 1 and the resistor 3 for limiting the current supplied from the power supply 4 to the primary winding 2a of the ignition coil 2 based on the voltage across the resistor 3. A resistor 9b has one end thereof connected to a junction between the resistor 9a and the current limiter 5, and the other end thereof to ground. The power transistor circuit 1 includes an NPN type power transistor 1a which has a collector connected to one end of the primary winding 2a of the ignition coil 2, and an emitter connected to one end of the resistor 3, and an NPN type switching transistor 1b which has a collector connected to the collector of the power transistor 1a, an emitter connected to a base of the power transistor 1a and a base connected through a resistor 9c to a junction between one end of a resistor 11, which is connected at the other end thereof to the power source 4, and a collector of a transistor 12. The transistor 12 has an emitter connected to ground and a base connected to a signal generator (not shown) which generates an ignition signal in synchronism with the rotation of an engine.
The overall construction of the known igniter I is shown in FIG. 7. In this figure, housed in a casing 6, is a hybrid integrated circuit board 7 (hereinafter referred to as an HIC board), which has a hybrid integrated circuit (hereinafter referred to as an HIC) comprising the resistors 9a-9c (not shown in FIG. 7), a conductive layer forming the current detecting resistor 3, an IC chip forming the current limiter 5 and the like formed on the surface thereof. The HIC board 7 has a plurality of terminals 7a-7f formed on the surface thereof which are electrically connected to the IC chip through interior circuits (not shown) formed therein. A connector 10 is integrally formed with the casing 6 and has a plurality of connector terminals 10a-10c which are connected to the terminals 7a-7c, respectively, on the HIC board 7. The HIC board 7 is fixedly secured through an elastomeric adhesive (not shown) to a bottom interior surface of the casing 6. The conductive layer 3 is formed of a thin metallic film and has one end thereof connected at the terminal 7e to the emitter of the power transistor 1a of the power transistor circuit 1, and the other end thereof connected at the terminal 7c to the connector terminal 10b which is adapted to be connected to ground when the connector 10 is coupled to an appropriate portion of an ignition apparatus which includes the elements 2, 4, 11 and 12 of FIG. 6. The other terminals of the power transistor circuit 1 are respectively connected to the corresponding terminals 7d, 7f on the HIC board 7.
In operation, when an ignition signal is supplied from the unillustrated signal generator to the base of the transistor 12, the transistor 12 is made conductive, thus turning on the power transistor circuit 1. As a result, a current flows from the power supply 4 to ground through the primary winding 2a of the ignition coil 2, the now conductive power transistor 1a and the resistor 3, so that a voltage is developed across the resistor 3 and supplied to the current limiter 5. Based on this voltage, the current limiter 5 controls the primary current flowing in the primary winding 2a of the ignition coil 2. In accordance with the primary current, a high voltage is developed across the secondary winding 2b of the ignition coil 2, and supplied to a distributor (not shown) for causing an appropriate spark plug to generate a spark.
With the known igniter I as constructed above, however, upon assembly thereof, it is necessary to manually secure the HIC board 7, which is very brittle and fragile, to the bottom interior surface of the casing 6 with an elastomeric adhesive using utmost care so as to prevent any damage thereto. In this connection, although the elastomeric adhesive serves to absorb stress which may be applied to the HIC board 7 when secured to the casing 6, such an assembly operation is rather troublesome and inefficient, adding to the cost of manufacture. In addition, in order to prevent any inadvertent damage to the IC chip formed on the surface of the HIC board 7 upon assembly thereof, it is necessary to provide a protective coating on the surface of the HIC board 7 covering the IC chip. Further, after having been secured to the casing 6, the HIC board 7 must be dried or held in a high-temperature atmosphere for a predetermined time (i.e., around one hour or so) until the adhesive solidifies. For these reasons, it is difficult to automate the entire process of manufacture.
Moreover, since a large primary current flows in the conductive layer 3 formed on the surface of the HIC board 7, the thin film-like conductive layer 3 must have sufficient length and width for providing a necessary resistance for current detection as well as allowing such a large primary current to flow. This results in an increase in the size of the HIC board 7 and hence the size of the entire igniter.